1. Field of the Invention
The present invention relates, in general, to a pulse generating apparatus for a xenon lamp. In particular, the invention relates to a pulse generating apparatus for a xenon lamp which is used in a motion picture projector. The invention also relates to a method for lighting a xenon lamp.
2. Description of the Prior Art
A xenon lamp is a kind of arc lamp. The luminous flux of the xenon lamp reaches a stable level immediately after lighting, and the luminance thereof is extremely high. Furthermore, the xenon lamp has good rendering properties, and the color temperature and the molecular spectral distribution thereof are similar to that of sunlight. A xenon lamp may be a short arc lamp or a long arc lamp. In general, since the long arc lamp has a large luminous energy, it is used for lighting a wide space. On the other hand, the short arc lamp is similar to a point light source. Such a short arc lamp often is used as a light source for a motion picture projector.
In conventional motion picture projectors using a xenon short art lamp as a light source, the xenon lamp remains on continuously, and the light therefrom is regularly interrupted by a shutter in synchronism with the frame feeding of a moviefilm. In this type of projector, however, a luminance change of the light is caused when the light from the lamp is interrupted. As a result, flickers occur on the moviescreen.
To reduce the flicker described above, a shutter device including two blades disposed opposite to one the other may be used for interrupting the light from the xenon lamp. If the frame feeding speed of the moviefilm is 24 frames/second, the shutter device is rotated at 24 times/second. Therefore, the light from the lamp is interrupted 48 times/second. This type of shutter device may reduce or eliminate the flicker on the screen. However, in this system wherein the light from the lamp is interrupted by the shutter, the xenon lamp remains lighted at a prescribed current value even when the shutter is closed. Therefore, the luminous efficiency of the lamp is adversely affected.
To solve the above problem, a shutterless system has been developed. One such shutterless system is disclosed in Japanese Utility Model publication No. 30134/1981 laid open on Sept. 22, 1977, and entitled ARC LAMP.
In this system, a prescribed minimum current, e.g., 1 A, is supplied to a xenon lamp for maintaining the arc of the lamp. Under this minimum current, no perceived illumination of the xenon lamp occurs. A large pulse current, e.g., 38 A, is overlaid on the minimum current to illuminate the xenon lamp at a prescribed interval. Therefore, the xenon lamp is frequently turned on and off in synchronism with the application of the pulse current. According to this system, effects similar to those of the above-described shutter system also may be achieved in this system. Furthermore a higher luminous efficiency is achieved in comparison with the shutter system. This is because a large pulse current is supplied only when the xenon lamp is turned on. However, in this shutterless system, since the current change between the minimum current and the pulse current overlaid on the minimum current is large, the temperature change of the electrodes of the xenon lamp also is large. As a consequence, deterioration of the electrodes of the lamp is caused, and the life of the lamp is adversely affected.